1. Field of the Invention
This invention relates generally to magnetic recording disk drives, and more particularly to a disk drive that includes a system for canceling the effects of rotational vibration.
2. Description of the Related Art
Magnetic recording hard disk drives use an actuator, typically a rotary voice-coil-motor (VCM) actuator, for positioning the read/write heads on the data tracks of the recording disks. The disk drive has a servo control system that receives a position error signal (PES) from servo positioning information read by the heads from the data tracks and generates a VCM control signal to maintain the heads on track and move them to the desired track for reading and writing of data.
Disk drives experience rotational vibration and disturbance forces during normal operation. These disturbances arise internally, such as from motion of the VCM actuator, as well as externally, such as from shocks to the frame supporting the disk drive or from the movement of other disk drives when the drives are mounted together in a disk array system.
Rotational vibration (RV) cancellation is a method that uses sensors (typically accelerometers) to detect rotational vibration and improve the PES by canceling the off-track motion induced by the rotational vibration. The RV sensor signal is input to a feedforward controller that creates a feedforward compensation signal that is summed with the control signal to the VCM actuator. However, the conventional RV cancellation method may degrade the PES when the RV sensor output includes large amounts of noise which are not related to the rotational vibration. This problem becomes more critical as the track pitch becomes more narrow with the increasing data density of disk drives. Source of noise in the RV sensor output may be electrical or sensor-related. Examples of sensor-related noise include non-rotational vibrations detected by the sensor's cross-axis sensitivity, and/or spurious signals generated as a result of physical distortion of the sensor itself. When the rotational vibration is detected by a RV sensor made up of a pair of linear accelerometers, the gain mismatch of the accelerometers also creates an undesirable compensation signal that degrades the PES.
What is needed is a reliable method to switch the RV cancellation off when the sensor detects primarily non-rotational vibration or noise and to switch the RV cancellation on when the sensor is detecting primarily rotational vibration.